This has "bugged" me for a LONG time, too.
The AISC Code (ASD or LRFD) goes out of its way to define itself legally as
a code for "design of buildings." NOTE: Not "Buildings and other
structures," but simply "Buildings." And the Code doesn't define the word
"building," interestingly enough.
The Preface to the latest edition of the AISC LRFD Code states "This
specification has been developed as a consensus document to provide a
uniform practice in the design of steel-framed buildings. The intention is
to provide design criteria for routine use and NOT TO PROVIDE SPECIFIC
CRITERIA FOR INFREQUENTLY ENCOUNTERED PROBLEMS, WHICH OCCUR IN THE FULL
RANGE OF STRUCTURAL DESIGN." [Emphasis mine]
Well, great. I daresay that many of us DO indeed use it for "infrequently
encountered problems" anyway. I admit to having done so in the past, before
my gradual enlightenment as to the legal as well as the engineering aspects
of what we are called upon to do.
And may I point out that notwithstanding this disclaimer, the code DOES
address certain "infrequently encountered problems," such as single-angle
member design and (such is my concern these days) design for fatigue.
Now, the code committee has obviously sought to sufficiently obscure its
intent in this wise, by sticking these "infrequently encountered problems"
in Appendices, leaving only some tantalizing clues in the main body of the
code to attract the diligent hunter of structural engineering arcana to his
prey. For example, fatigue design is briefly mentioned in Chapter K of the
AISC LRFD code, where we meet this bit of foggy elucidation: "Few members of
connections in CONVENTIONAL BUILDINGS need to be designed for fatigue, since
most load changes in such structures occur only a small number of times or
produce only minor stress fluctuations. ...However, crane runways and
supporting structures for machinery and equipment are often subject to
fatigue loading conditions." [Emphasis mine]
In other words "you don't really need to consider fatigue criteria in
FREQUENTLY ENCOUNTERED PROBLEMS (which is manifestly what the Code is meant
to address) but when you have one of those INFREQUENTLY ENCOUNTERED
PROBLEMS--you know, the kind the Code is NOT meant to address, we refer you
to Appendix K3."
So, here we go to Appendix K3, where an absolutely SPLENDID array of tools
is given to solve a wide variety of structural problems related to fatigue
that one might INFREQUENTLY ENCOUNTER. In fact, it seems made to order for
the problem that I have, which is a rib-stiffened steel plate that I propose
to use to cover a ground vault under a highway, so that it won't be pounded
to pieces when 400+ heavy transit buses per day roll over it over the next
I'm here to tell you, those fatigure provisions are MADE-TO-ORDER.... Er!
Except that I don't have a BUILDING, do I? I have a piece of structural
flotsam (or jetsam, I'm really not sure which) cast out onto a major highway
traffic lane, and I have to come up with a way to justify the design
Now, to be sure, I'm also making reference to the AASHTO LRFD Specification
for Highway Bridge Design. That gives me the notional loads that I need, in
particular the wheel load and footprint thereof (since this is a small bit
of flotsam or jetsam, the load footprint is actually a significant aspect of
design). And that specification has some information about fatigue design as
well, much of which correlates--to an extent--to that found in AISC.
That code is for HIGHWAY BRIDGES. It is prescriptive in many respects, and
many of the requirements therein simply don't apply to my little metal
For example, it does not recognize ANY plate material other than ASTM A 709,
which is bridge steel. I suppose I can make my plate out of bridge steel,
but why not more easily obtained, and cheaper, ASTM A 572 or similar (this
thing is small, but this is a prototype design that will be fabricated over
and over and over again, maybe two or three hundred of them scattered all
around Harris County, Texas) to make a more economical design.
Also, some of the requirements of bridges don't apply. For example, do I
reference the requirements of the AWS D1.5 Highway Bridge Welding Code? That
requires a bunch of things like Charpy testing of welds, etc., that will be
prohibitively expensive (not to mention unnecessary) for my application.
In short, because AISC is for BUILDINGS--even though the provisions thereof
are general in nature--and because AASHTO is for BRIDGES--with restrictive
prescriptive requirements--I'm sort of hung out to dry as far as legal cover
for what I am about to do.
That's the thing: If something were to happen to one of these covers (it
would only take one, I think) how do I justify my design approach, if I base
it on AISC which is "for buildings"? Even though I know (well, I THINK I
know this) that the provisions therein are adaptable to what I am trying to
How is our litigious society disposed to view my "right" as an engineer to
use my "engineering judgement" to effect a solution to such an "INFREQUENTLY
ENCOUNTERED PROBLEM"? Especially if I'm wrong?
William L. Polhemus, Jr., P.E.
Polhemus Engineering Company
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